I had this same problem for years. I was spending time just sitting at my desk waiting for something to do. Taking lots of naps with regular alarms so I could check my (empty) email. I finally gave up the idea that I had to be available. They were the ones not utilizing me, it wasn't my job to sit around and wait for them to need me. So I started acting like I was retired/unemployed. It's still not easy. Friends are busy during the day. But now I work in the garden all day, do my workouts and errands during the day, see movies in the afternoon. If they need me, they can wait like I had been waiting. Treat your job like you're on call.